* {
    margin: 0;

    padding: 0;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-image: linear-gradient(to top , #141e30,#243b55);
}

.login {
    display: flex;
    /* 此方法改变了弹性盒子的主轴方向，让盒子的元素的排列方式发生了改变 */
    flex-direction: column;
    align-items: center;
    background-color: rgba(0,0,0,0.2);
    width: 400px;
    padding: 40px;
    box-shadow: 0 15px 25px rgba(0,0,0,0.4);
}

.login h2 {
    color: #fff;
    margin: 30px;
}

.login .login_box {
    position: relative;
    width: 100%;
}

.login .login_box input {
    outline: none;
    border: none;
    width: 100%;
    padding: 10px 0;
    margin-bottom: 30px;
    color: #fff;
    font-size: 16px;
    border-bottom: 1px solid #fff;
    background-color: transparent;
}

.login .login_box label {
    position: absolute;
    top: 0;
    left: 0;
    padding: 10px 0;
    color: #fff;
    /* 此属性默认是auto 默认可以被点击，如果写成none的话，将不可选取 */
    pointer-events: none;
    transition: all 0.5s;
}
/* focus选择器是当input获得焦点时,valid选择器是当内容合法时，当获得焦点时并且有输入时执行下面的样式 */
.login .login_box input:focus + label,
.login .login_box input:valid + label {
    top: -20px;
    color: #03e9f4;
    font-size: 12px;
}

.login a {
    overflow: hidden;
    position:relative;
    padding: 10px 20px;
    color: #03e9f4;
    text-decoration: none;
    /* transition过度语句 */
    transition: all 0.5s;
}

.login a:hover {
    color: #fff;
    border-radius: 5px;
    background-color: #03e9f4;
    box-shadow: 0 0 5px #03e9f4,
    0 0 25px #03e9f4,
    0 0 50px #03e9f4,
    0 0 100px #03e9f4;
}

.login a span {
    position: absolute;
}

.login a span:first-child {
    top: 0;
    left: -100%;
    width: 100%;
    height: 2px;
    background-image: linear-gradient(to right,transparent,#03e9f4);
    animation: move1 1s linear infinite;
}

.login a span:nth-child(2) {
    top: -100%;
    right: 0;
    width: 2px;
    height: 100%;
    background: linear-gradient(transparent,#03e9f4);
    animation: move2 1s linear 0.25s infinite;
}

.login a span:nth-child(3) {
    bottom: 0;
    right: -100%;
    height: 2px;
    width: 100%;
    background: linear-gradient(to left,transparent,#03e9f4);
    animation: move3 1s linear 0.5s infinite;
}

.login a span:last-child {
    top:100%;
    left: 0;
    width: 2px;
    height: 100%;
    background: linear-gradient(to top,transparent,#03e9f4);
    animation: move4 1s linear 0.75s infinite;
}

@keyframes move1 {
    0% {
        left: -100%;
    }
    50%,100% {
        left: 100%;
    }
}

@keyframes move2 {
    0% {
        top: -100%;
    }
    50%,
    100% {
        top: 100%;
    }
}

@keyframes move3 {
    0% {
        right: -100%;
    }
    50%,
    100% {
        right: 100%;
    }
}

@keyframes move4 {
    0% {
        top: 100%;
    }
    50%,
    100% {
        top: -100%;
    }
}